This invention relates generally to a process for producing reinforced polyester products employing a pultrusion process and to polyester resin compositions used therein. More particularly, it is directed to a pultrusion process that is run at significantly greater pulling or running speeds, and to particular new polyester resin compositions that make possible the use of such higher speeds.
Pultrusion is a newer process of producing reinforced plastic products than are molding or hand-lay-up methods having its origin in the fifties. Generally, the term "pultrusion" is used to describe any process of producing reinforced plastics in which continuous reinforcing material is impregnated with resin and pulled through a die of desired cross section to shape and cure the resin and produce continuous lengths of cured product having a uniform cross section and the shape of the die. Thermosetting-type resins are used almost exclusively at the present time with polyester resins comprising 85 to 90 percent of the total and epoxies the rest.
In pultrusion, the reinforcing material may be any filamentary material having strength, such as glass fiber, Aramid fibers, boron fibers or graphite fibers. Cusomarily, E-glass fibers constitute the majority of reinforcing material used. Most commonly, filamentary reinforcing materials are used in the forms of: rovings, tows, mats, or cloth, or combinations of these forms.
Because pultrusion incorporates continuous strands of reinforcing material, it produces a product much stronger and flexible than those produced by older extrusion processes, which can only utilize discontinuous lengths of reinforcing filament material. Because of these characteristics, whenever high-strength products having the customary properties of polyester resins are desired, such as corrosion resistance, electrical resistance or lightweight, then pultrusion will be used. The sizes and shapes of products, either solid or hollow, produced by pultrusion is virtually unlimited, as for example, I-beams, channel, wide-flange beams, solid bars and rods, round and square tubing, rectangular beams, angles, and even flat sheets. Customarily, the thickness of these parts can range from as little as 1/8 inch up to a practical maximum of about three inches.
Initially, pultrusion did not grow rapidly because it was costly and was limited to products with small cross-sectional areas. During the 1950's, however, radio frequency (RF) preheating was developed, which permitted faster line speeds and larger cross-sectional products to be made. However, even with RF-augmented heating, there is a practical limit to pultrusion speed beyond which pultruded parts exhibit either internal thermocracking due to excessive exotherm or external or internal cracking due to undercuring and monomer gassing. Typically, products having a thickness of about 0.25-0.375 inch made with relatively fast-curing polyester resins can be pultruded at a maximum speed of about 8 to 9 feet per minute with fairly smooth surfaces; products having a thickness of about 0.375 to 0.675 inch made with an intermediate reactivity resin can be pultruded at speeds up to 6 feet per minute with only mildly abraded surfaces; while thick products over 0.67 inches in thickness and made with medium reactivity resins are limited to speeds of about 0.5-5 feet maximum depending on thickness and have pronounced surface abrasion and roughness. Additionally, because of the thermal cracking that occurs when the curing exotherm is excessive, thick parts generally can only be made with medium reactivity resins having a maximum degree of unsaturation of about 50 to 55 mole percent of the total diacids (100 mole percent) employed in producing the resin. Because of their limited reactivity, these resins tend to give softer cured surfaces that are more prone to be abraded in the die and to give products having rough, uneven surfaces.